The present invention relates to an illumination type optical recorded information reading device having an illuminating means such as a light emitting element for illuminating a media on which optical information such as a bar code is recorded and having a photo detecting element for reading the optical information and converting the same into electrical information. More particularly, it relates to such an illumination type optical recorded information reading device improved in illumination characteristic and reading accuracy.
Conventionally known is an optical information reading device having a line sensor for reading a bar code recorded on a media such as a label, a binary coder for binary-coding an output from the line sensor, and a decoder for decoding a binary-coded output from the binary coder, thereby decoding the bar code. The optical information reading device is provided with a light source such as an LED array for illuminating a region of the bar code on the bar code media. A reflected light from the bar code media is received by the line sensor to read the bar code.
FIG. 15 shows an example of a light source driving circuit employed in the conventional optical information reading device. As shown in FIG. 15, the optical information reading device includes a plurality of LEDs (light emitting diodes) 1 to 10 and a plurality of resistors 11 to 15. The LEDs 1 and 2 and the resistor 11 are connected in series; the LEDs 3 and 4 and the resistor 12 are connected in series; the LEDs 5 and 6 and the resistor 13 are connected in series; the LEDs 7 and 8 and the resistor 14 are connected in series; and the LEDs 9 and 10 and the resistor 15 are connected in series. Each series circuit consisting of the two LEDs and the one resistor is supplied with a DC supply voltage V.sub.cc from a single common power supply. With this construction, currents flow through the LEDs 1 to 10, and light is emitted from the LEDs 1 to 10.
All the LEDs 1 to 10 have the same characteristic, and all the resistors 11 to 15 have the same resistance. The resistors 11 to 15 serve to limit the amperage of the currents flowing through the LEDs 1 to 10, and the LEDs 1 to 10 are driven to emit the light having an intensity corresponding to the amperage of the currents flowing therethrough.
The LEDs 1 to 10 constitute a light source for illumination in the optical information reading device, and they are so arranged as to uniformly illuminate a region of the bar code on the bar code media.
The optical information reading device further includes a photo detecting means for detecting a reflected light from the region of the bar code on the bar code media. The photo detecting means is constructed of a line sensor having a plurality of photo detecting elements arranged in rows and a reduction optical system such as a lens for condensing the reflected light from the media. The optical information recorded on the media is converted into electrical information by the photo detecting elements, and the electrical information is sequentially input to a binary coder, in which a waveform of the electrical signal is shaped to be processed for a subsequent operation in a computer.
Such a prior art optical reading device will be described with reference to FIGS. 17 and 18A to 18D. FIG. 17 schematically shows the arrangement of an optical system in the optical reading device, and FIGS. 18A to 18D show an operation of the optical reading device shown in FIG. 17.
Referring to FIG. 17, a light source 21 constructed of a plurality of LEDs as the light emitting elements arranged in rows emits light as shown by five parallel arrows to illuminate a media 22 in a predetermined range where optical information is indicated by bars and spaces. A reflected light scattered from the surface of the media 22 is transmitted through a lens 25 as the reduction optical system to reach a line sensor 23.
In the case that an illuminance of the light emitted from the light source 21 is uniform in a detectable range of the media 22 by the line sensor 23, and that a reflectance on the surface of the media 22 is constant, a distribution of an illuminance of the light detected by the line sensor 23 is shown in FIG. 18B, wherein a reflected light quantity from the media 22 is gradually reduced from a central portion of a detected range of the line sensor 23 to opposite ends thereof. To make the reflected light quantity uniform over the entire detected range of the line sensor 23, an illuminance distribution in the detectable range on the media 22 is modified as shown in FIG. 18A, wherein the illuminance in the detectable range is gradually increased from the central portion of the detectable range to the opposite ends thereof. Such a modification of the illuminance distribution can be realized by changing a pitch of arrangement of the LEDs or changing an amperage of the currents flowing through the LEDs as described in Japanese Patent Publication No. 62-17270, for example. Accordingly, the reflected light quantity from the media 22 to be detected by the line sensor 23 in the detected range can be made uniform as shown in FIG. 18C, provided that the reflectance on the surface of the media 22 is constant.
In the case that the bar code is present in the detectable range on the media 22, when a boundary between a non-indicated region where the bar code is not indicated and an indicated region where the bar code is indicated is detected by the line sensor 23, an output signal having a large amplitude is generated from the line sensor 23 as shown by "E" in FIG. 18D. Subsequently when the bar code is continuously detected by the line sensor 23, an output signal having a repeated amplitude as shown by "e" in FIG. 18D is generated from the line sensor 23 in correspondence with the bars and the spaces of the bar code. Such an analog waveform signal is supplied through a waveform shaping circuit to a computer in which the signal is processed.
An example of the above-mentioned illumination type optical recorded information reading device is disclosed in U.S. Pat. No. 4,528,444.
In the optical information reading device for reading the bar code, a dry cell is used as the power supply for the purpose of improvement in transportability and easiness of handling. However, an output voltage of the dry cell changes with a service duration and a peripheral environment. FIG. 16 shows a test result of an aged deterioration of the output voltage of the dry cell. The test was carried out under the condition where a discharge load current was set to 150 mA, and the operation of discharging for 5 seconds and stopping the discharging for 60 seconds was repeated at an environmental temperature of 25.degree. C. In FIG. 16, a curve (a) represents a terminal (output) voltage of the dry cell just before the discharging, and a curve (b) represents a terminal voltage of the dry cell just after the discharging.
It is apparent from FIG. 16 that the output voltage of the dry cell is lowered as the service duration proceeds, and that the output voltage on the curve (a) is lowered to lie on the curve (b) after the discharging for 5 seconds, and it is restored to lie on the curve (a) after the stopping of the discharging for 60 seconds. A fluctuation of the output voltage due to the discharging is about 0.1 V even at the beginning of the service duration of the dry cell (i.e., the service duration=0), and the fluctuation increases to about 0.3 V after the service duration of 17 hours near a service life of the dry cell. Further, such a fluctuation occurs in a single dry cell, and if a plurality of dry cells are connected in series for service, the fluctuation is further increased in correspondence with the number of the dry cells.
In the light source driving circuit shown in FIG. 15, when the supply voltage V.sub.cc is generated from a dry cell, the amperage of the currents flowing through the LEDs 1 to 10 is changed because the supply voltage V.sub.cc from the dry cell changes for the above reason, and the resistance of the resistors 11 to 15 is constant. As a result, a light emission quantity from the LEDs 1 to 10 is changed. Further, even when the output voltage (supply voltage V.sub.cc) of the dry cell is low, the light source driving circuit can be operated by reducing the resistance of the resistors 11 to 15. However, such a reduction in the resistance of the resistors 11 to 15 causes an increase in a rate of fluctuation in the amperage of the currents flowing through the LEDs 1 to 10 with respect to the fluctuation in the output voltage of the dry cell.
This phenomenon will now be explained in the series circuit of the resistor 11 and the LEDs 1 and 2 by way of example. Letting R denote the resistance of the resistor 11, V.sub.F denote a voltage between an anode and a cathode of each of the LEDs 1 and 2, and I denote an amperage of the current flowing through the LEDs 1 and 2, the amperage I is given as follows: EQU I=(V.sub.cc -2V.sub.F)/R (1)
where the voltage V.sub.F is set to 1.7 V, and the amperage I is set to 20 mA. When the supply voltage V.sub.cc is 12 V, the resistance R is obtained from the equation (1) as follows: EQU R=(12-1.7.times.2)/(20.times.10.sup.-3)=430.OMEGA. (2)
when the resistance R of the resistor 11 is set to 430.OMEGA., and the supply voltage V.sub.cc changes from 12 V by 10% (i.e., 10.8 to 13.2 V), the amperage I changes in the range of 17.21 to 22.79 mA. That is, the amperage I changes by .+-.14%.
In contrast, when the supply voltage V.sub.cc is 5 V, and the amperage I is set to 20 mA, the resistance R becomes 8.OMEGA. in the same manner as the above. In this case, when the supply voltage V.sub.cc changes from 5 V by 10% (i.e., 4.5 to 5.5 V), the amperage I changes in the range of 13.75 to 26.25 mA. That is, the amperage I changes by .+-.31.25%. Thus, the rate of the fluctuation in the amperage is larger when the supply voltage is low than when it is high.
As mentioned above, when the supply voltage V.sub.cc is fluctuated, the amperage of the currents flowing through the LEDs 1 to 10 to result in a fluctuation in light emission quantity from the LEDs 1 to 10. Furthermore, when the resistance of the resistors 11 to 15 is set to a small value, the rate of the fluctuation in light emission quantity from the LEDs 1 to 10 with respect to the fluctuation in supply voltage is large.
Such a fluctuation in light emission quantity from the LEDs 1 to 10 causes a fluctuation in reflected light quantity from the bar code media to the line sensor, which in turn causes a fluctuation in amplitude of the output signal from the line sensor. As a result, there is a possibility that the output signal from the line sensor is not properly binary-coded in the binary coder circuit, thus causing a decoding error of the bar code.
The above problem is considered to be solved by obtaining the supply voltage V.sub.cc from a voltage regulator circuit. However, the voltage V.sub.F between the anode and the cathode of each of the LEDs 1 to 10 is not actually equal to each other. In the case of using LEDs emitting a red light, for example, the voltage V.sub.F varies in the range of 1.68 to 1.85 V. Accordingly, even when the resistance of the resistors 11 to 15 is made equal to each other with a high accuracy, there is generated a dispersion of the currents flowing through the LEDs 1 to 10, thus causing a dispersion of the light emission intensity of the LEDs 1 to 10.
Further, in the prior art optical reading device as shown in FIG. 17, an indication start position and an indication end position of the bar code or the like are located in the vicinity of the opposite ends of the detectable range on the media 22. In the case that the boundary between the indicated region (ranging from the indication start position to the indication end position) and the non-indicated region is detected, an average amplitude level of the analog signal waveform from the line sensor 23 becomes larger than that when the indicated region (the bar code) is detected. As a result, it is difficult to accurately process the waveform in the waveform processing circuit, resulting in an increase in reading error.